Search

Over 317,000 tonnes of road salt (NaCl) are applied annually for road deicing in the Twin Cities Metropolitan Area (TCMA) of Minnesota. Although road salt is applied to increase driving safety, this practice influences environmental water quality. Thirteen lakes in the TCMA were studied over 46 months to determine if and how they respond to the seasonal applications of road salt. Sodium and chloride concentrations in these lakes were 10 and 25 times higher, respectively, than in other non-urban lakes in the region. Seasonal salinity/chloride cycles in the lakes were correlated with road salt applications: High concentrations in the winter and spring, especially near the bottom of the lakes, were followed by lower concentrations in the summer and fall due to flushing of the lakes by rainfall runoff. The seasonal salt storage/flushing rates for individual lakes were derived from volume-weighted average chloride concentration time series. The rate ranged from 9 to 55% of a lake''s minimum salt content. In some of the lakes studied salt concentrations were high enough to stop spring turnover preventing oxygen from reaching the benthic sediments. Concentrations above the sediments were also high enough to induce convective mixing of the saline water into the sediment pore water. A regional analysis of historical water quality records of 38 lakes in the TCMA showed increases in lake salinity from 1984 to 2005 that were highly correlated with the amount of rock salt purchased by the State of Minnesota. Chloride concentrations in individual lakes were positively correlated with the percent of impervious surfaces in the watershed and inversely with lake volume. Taken together, the results show a continuing degradation of the water quality of urban lakes due to application of NaCl in their watersheds. [Copyright &y& Elsevier]

Summary: Stream flow records (up to the year 2002) from 36 USGS gauging stations in five major river basins of Minnesota were studied. Seven annual stream flow statistics were extracted and analyzed: mean annual flow, 7-day low flow in winter, 7-day low flow in summer, peak flow due to snow melt runoff, peak flow due to rainfall as well as high and extreme flow days (number of days with flow rates greater than the mean plus one or two standard deviations, respectively). The Mann–Kendal non-parametric test was used to detect significant trends over time windows from 90 to 10 years in combination with the Trend Free Pre-Whitening (TFPW) method for correcting time series data for serial correlation. Streamflows in the state of Minnesota have varied over the period of record. Trends differed significantly from one river basin to another, and became more accentuated for shorter time windows. Periodicity was detected in the trends for the Red River of the North, the Mississippi River, and the Minnesota River basins for six of the statistics studied. Periods were on the order of 13–15 and 25 years, and the amplitudes were particularly strong after 1980. Peak flow due to snowmelt, typically the highest flow in each year, appears to be the only streamflow statistic that has not changed at a significant rate. Peak flows due to rainfall events in the summer are increasing, as well as the number of days with higher flows (high flow days). Increases in low flow (base flow) in summer and in winter have been significant. Wetter summers and more frequent snow melt events due to warmer winters are the likely cause. Stream flows in Minnesota reflect observed changes in precipitation with increases in mean annual precipitation, a larger number of intense rainfall events, more days with precipitation and earlier and more frequent snowmelt events. For water resources management the results suggest that the threat of snowmelt flooding has not increased, but floods due to rainfall events are more likely. Higher summer and winter base flows may benefit water quality. [Copyright &y& Elsevier]